5.7.4.3. Spinlocks

• fast, busy waiting mutual exclusion mechanism

• Mechanism:
  • Atomic variable is initalized to 1: available
  • spin_lock() atomically decrements value and checks if equals 0
    • if yes: lock obtained
    • if no: no lock, spin in tight loop until value is 1
  • spin_unlock() sets value back to 1

• must only be used in code that cannot sleep!
  • no userspace memory access
  • no memory allocation (except GFP_ATOMIC)

• three flavors
  • spin_lock(), spin_unlock()
    • on UP, no preemption: optimized out
    • on UP with preemption: disable preemption
    • protects against concurrency with "regular" kernel code
  • spin_lock_irqsave, spin_unlock_irqrestore, spin_lock_irq, spin_unlock_irq
    • disables interrupts on local CPU
    • protects against concurrency with interrupt handlers
  • spin_lock_bh, spin_unlock_bh
    • disables soft interrupt on local CPU
    • protects against concurrency with bottom halves, tasklets, softirqs, ...

• Advantages:
  • fast and simple

• Disadvantages:
  • code must be atomic: can never sleep!
  • unfair! solution: ticket spinlocks (merged 2.6.25)